bin: handle thousands of output files

Fixes #534.  Test case is only on next branch.

* bin/common_file.hh, bin/common_file.cc: Make it possible
to reopen a closed file.
* bin/common_output.cc, bin/common_aoutput.cc: Add a heuristic
to decide when to close files.
* NEWS: Mention the issue.

NEWS

@@ -2,6 +2,13 @@ New in spot 2.11.5.dev  (not yet released)
 
   Nothing yet.
 
+  Bug fixes:
+
+  - Running command lines such as "autfilt input.hoa -o output-%L.hoa"
+    where thousands of different filenames can be created failed with
+    "Too many open files".  (Issue #534)
+
+
 New in spot 2.11.5  (2023-04-20)
 
   Bug fixes:

bin/common_aoutput.cc

@@ -636,7 +636,27 @@ automaton_printer::print(const spot::twa_graph_ptr& aut,
       auto [it, b] = outputfiles.try_emplace(fname, nullptr);
       if (b)
         it->second.reset(new output_file(fname.c_str()));
+      else
+        // reopen if the file has been closed; see below
+        it->second->reopen_for_append(fname);
       out = &it->second->ostream();
+
+      // If we have opened fewer than 10 files, we keep them all open
+      // to avoid wasting time on open/close calls.
+      //
+      // However we cannot keep all files open, especially in
+      // scenarios were we use thousands of files only once.  To keep
+      // things simple, we only close the previous file if it is not
+      // the current output.  This way we still save the close/open
+      // cost when consecutive automata are sent to the same file.
+      static output_file* previous = nullptr;
+      static const std::string* previous_name = nullptr;
+      if (previous
+          && outputfiles.size() > 10
+          && &previous->ostream() != out)
+        previous->close(*previous_name);
+      previous = it->second.get();
+      previous_name = &it->first;
     }
 
   // Output it.

bin/common_file.cc

@@ -44,13 +44,30 @@ output_file::output_file(const char* name, bool force_append)
   os_ = of_.get();
 }
 
+void
+output_file::reopen_for_append(const std::string& name)
+{
+  if (of_ && of_->is_open())     // nothing to do
+    return;
+  const char* cname = name.c_str();
+  if (cname[0] == '>' && cname[1] == '>')
+    cname += 2;
+  if (name[0] == '-' && name[1] == 0)
+    {
+      os_ = &std::cout;
+      return;
+    }
+  of_->open(cname, std::ios_base::app);
+  if (!*of_)
+    error(2, errno, "cannot reopen '%s'", cname);
+}
 
 void output_file::close(const std::string& name)
 {
   // We close of_, not os_, so that we never close std::cout.
   if (os_)
     os_->flush();
-  if (of_)
+  if (of_ && of_->is_open())
     of_->close();
   if (os_ && !*os_)
     error(2, 0, "error writing to %s",

bin/common_file.hh

@@ -37,6 +37,8 @@ public:
 
   void close(const std::string& name);
 
+  void reopen_for_append(const std::string& name);
+
   bool append() const
   {
     return append_;

bin/common_output.cc

@@ -420,7 +420,27 @@ output_formula_checked(spot::formula f, spot::process_timer* ptimer,
       auto [it, b] = outputfiles.try_emplace(fname, nullptr);
       if (b)
         it->second.reset(new output_file(fname.c_str()));
+      else
+        // reopen if the file has been closed; see below
+        it->second->reopen_for_append(fname);
       out = &it->second->ostream();
+
+      // If we have opened fewer than 10 files, we keep them all open
+      // to avoid wasting time on open/close calls.
+      //
+      // However we cannot keep all files open, especially in
+      // scenarios were we use thousands of files only once.  To keep
+      // things simple, we only close the previous file if it is not
+      // the current output.  This way we still save the close/open
+      // cost when consecutive formulas are sent to the same file.
+      static output_file* previous = nullptr;
+      static const std::string* previous_name = nullptr;
+      if (previous
+          && outputfiles.size() > 10
+          && &previous->ostream() != out)
+        previous->close(*previous_name);
+      previous = it->second.get();
+      previous_name = &it->first;
     }
   output_formula(*out, f, ptimer, filename, linenum, prefix, suffix);
   *out << output_terminator;